1. Field of the invention
The present invention concerns protected transmission of digital signals, with particular reference to microwave transmission.
2. Description of the prior art
The object of the protection is to make the link resistant to jamming; one well-known protection method is the direct sequence spread spectrum method, to which the invention relates.
This method is described, for example, in "Coherent Spread Spectrum Systems" by Jack K. Holmes, a Wiley-Interscience Publication, John Wiley & Sons, 1982.
Essentially, the method consists in combining the information symbol to be transmitted with a higher information rate pseudo-random sequence known to the transmitter and to the receiver. The converse operation is carried out at the receiving end so as to restore the original information symbols used in the transmission or, at worst, an estimate of said symbols (given that the transmitted message may be degraded by noise and by jamming).
The information symbols are complex symbols obtained by QPSK modulation and therefore comprising a real part and an imaginary part each on one or more bits.
These complex symbols are transmitted as a continuous stream at a information rate D.sub.s, as shown in FIG. 1.
The direct sequence spread spectrum process consists in producing at the transmitting end, at the same time as the information symbols, (complex) elements C.sub.j, known in the art as "chips", of a recurrent
pseudo-random sequence. The information rate D.sub.c of the chips is higher than the information rate D.sub.s of the symbols, the ratio r=D.sub.c /D.sub.s being referred to as the "spread gain" (for convenience, an integer value is usually chosen for r). The complete sequence comprises L chips, in other words a sequence of L/r symbols. Immediately the sequence of chips C.sub.j is completed it is repeated to be combined with the subsequent symbols.
The result of combining the symbols a.sub.i and the chips C.sub.j will be a stream of symbols s.sub.i at the information rate D.sub.c of the chips and this stream modulates the communication transmitter.
The transposition from the information rate D.sub.s to the higher information rate D.sub.c results in a spreading of the transmitted signal spectrum in the frequency domain, in other words the frequency band used is r times wider than the Nyquist band for transmitting the information symbols alone.
This results in:
increased noise immunity (by a factor approximating the spread gain), PA1 excellent resistance to jamming, as the effects of jamming are limited to a restricted part of the spectrum, given the spreading of the spectrum, and PA1 reduced detectability (the same transmitted energy is spread over a wider band and the transmission spectral density is therefore much closer to the noise spectral density). PA1 firstly, it will not be necessary to filter out these special symbols in the stream of information symbols received; this stream can therefore be used as it is, without requiring any special preliminary processing; PA1 secondly, and more importantly, the sequence change can be detected at a very advanced stage of the process (for example: directly after demodulation), enabling immediate response even before the symbols have been decoded and processed). PA1 at the transmitting end, a pseudo-random sequence of successive complex elements is produced at a given information rate and each is multiplied by a complex information symbol arriving concurrently at a lower information rate to obtain a resultant sequence of complex elements modulating a carrier, PA1 at the receiving end, the complex elements obtained at the output of a demodulator are multiplied by a sequence of successive complex elements made up of elements conjugate to homologous elements used at the transmitting end, the transmit and receive sequences having been synchronized beforehand, and over the duration of an information symbol the resulting complex elements are summed to obtain an estimate of the original information symbols used at the transmitting end, PA1 at the transmitting end, one or other of two adjacent complex sequences of elements differing only with respect to intrinsic marking is used selectively according to whether the next sequence that will be used will or will not be the same as the current sequence, and PA1 at the receiving end, the complex elements obtained at the output of said demodulator are analyzed to identify said marking and a decision is taken to retain the same sequence of conjugate complex elements or to change sequence according to the marking identified, successive changes of sequence occurring at random times determined by the receiver exclusively on the basis of analyzing said markings but in accordance with a deterministic order known intrinsically by the receiver.
At the receiving end, the symbols s.sub.i obtained from the output of the demodulator have applied to them the operation which is the converse of that carried out at the transmitting end: to this end, the symbols s.sub.i received at the information rate D.sub.c are multiplied by the conjugates C.sub.i * of the elements C.sub.j of the sequence used for transmission, also generated at the information rate D.sub.c, and r successive results are then summed; this produces at the information rate D.sub.s =D.sub.c /r an estimate of the original information symbols a.sub.i (if there is no noise and no jamming these symbols are restored exactly).
This assumes, of course, that transmission and reception have been synchronized beforehand, so that the times at which the successive symbols start can be recognized.
The decoding process reduces the spread band to its original width and conversely spreads the jamming in a ratio r (the samples of the jamming signal will be multiplied by the conjugates of the elements of the sequence), so that its spectral density is strongly reduced which strongly reduces its disturbing effect.
This system achieves excellent protection against jamming, but it has the drawback that if the same sequence is used all the time there is an increased risk of interception and jamming; this is the case especially if the sequence is generated in a simple way from a small number of basic chips (a PN sequence of length L=1 023 is generated from a key of only 10 bits, for example): it will then be simple for an enemy to reconstitute this sequence and to intercept or jam the link.
Provision is usually made for changing the sequence during transmission to protect against this risk.
Until now, in all the techniques proposed for achieving this aim it has been necessary to advise the receiver of the impending change, either by interrupting the transmission of information and sending a message warning of the sequence change or by using a parallel channel separate to that used to transmit the information proper to transmit the sequence change information to the receiver.
These solutions are costly to implement (requiring an additional channel or a two-way link, management of link interruption, etc), which has so far limited their general adoption for this type of transmission.
One object of the invention is to propose a direct sequence spread spectrum communication system employing sequence changing during transmission which makes it possible, with a simple one-way link, to transmit sequence change information to the receiver during transmission without degrading or interrupting said link at the time of the change.
Another object of the invention is to propose a method of this kind achieving total transparency of the information transmitted, that is to say a method in which the information rate of the information is not modified and in which special symbols interleaved between the information symbols proper are not used to signal the change.
To this end, the invention proposes the use of intrinsic (that is to say, without additional special symbols) marking of the transmitted sequence; the receiver will then identify the type of marking and will take a decision accordingly.
Avoiding the use of special symbols has two advantages:
Thus with a technically simple device and a simple one-way link with no special modifications, it is possible to change sequence immediately, at random times decided on by the transmitter alone, and with no sign on the link (interruption, transmission of parallel signals, etc) giving any warning of an imminent sequence change to any person attempting to intercept the link.
The security of the link will therefore be increased very significantly, and with minimum equipment costs.